The continual demand for enhanced integrated circuit performance has resulted in, among other things, a dramatic reduction of semiconductor device geometries, and continual efforts to optimize the performance of every substructure within any semiconductor device. A number of improvements and innovations in fabrication processes, material composition, and layout of the active circuit levels of a semiconductor device have resulted in very high-density circuit designs. Increasingly dense circuit design has not only improved a number of performance characteristics, it has also increased the importance of, and attention to, semiconductor material properties and behaviors.
The increased packing density of the integrated circuit generates numerous challenges to the semiconductor manufacturing process. Every device must be smaller without damaging the operating characteristics of the integrated circuit devices. High packing density, low power consumption and good reliability must be maintained without any functional device degradation. Increased packing density of integrated circuits is usually accompanied by smaller feature size.
As integrated circuits become denser, the widths of interconnect layers that connect transistors and other semiconductor devices of the integrated circuit are reduced. As the widths of interconnect layers and semiconductor devices decrease, their resistance increases. As a result, semiconductor manufacturers seek to create smaller and faster devices by using, for example, a copper interconnect instead of a traditional aluminum interconnect. Unfortunately, copper is very difficult to etch in most semiconductor process flows. Therefore, damascene processes have been implemented to form copper interconnects.
Damascene methods usually involve forming a trench and/or an opening in a dielectric layer that lies beneath and on either side of the copper-containing structures. Once the trenches or openings are formed, a blanket layer of the copper-containing material is formed over the entire device. The thickness of such a layer must be at least as thick as the deepest trench or opening. After the trenches or openings are filled with the copper-containing material, the copper-containing material over them is removed, e.g., by chemical-mechanical planarization (CMP), so as to leave the copper containing material in the trenches and openings but not over the dielectric or over the uppermost portion of the trench or opening.
During CMP, slurry is applied to the surface of a wafer as a mechanical polishing pad polishes that surface. Slurries having a variety of chemical and physical compositions are available. Depending upon its composition and its intended use, slurry may be produced to be more copper selective, more dielectric selective, or material neutral. Regardless of their affinity or neutrality, however, slurries generally comprise a number of components, such as abrasives (e.g., alumina) and reactants (e.g., peroxides). At some point in time, these components are mixed together for slurry application to a wafer surface. Ideally, the resulting mixture typically comprises some homogeneous or quasi-homogeneous blend of various particulate matter and chemicals (e.g., suspension, emulsion).
There are generally two approaches to mixing slurry components—premixing and point-of-use mixing. Conventionally, each approach has a number of disadvantages associated therewith. For example, premixed slurries are mixed at a location apart from the CMP apparatus and, for some amount of time, must be stored. Furthermore, depending upon where and when such slurry has been premixed, it may require transport to a site where the CMP apparatus is. Where slurries are left unused while sitting in storage or transport, even for relatively small amounts of time (e.g., 1 hour), a certain degree of degradation begins. Agglomerations of slurry material (e.g., gels) may begin to form. Particulate matter may begin to settle. These and other related phenomena degrade the consistency and efficacy of the slurry over time—resulting in inconsistent CMP results wafer-to-wafer and lot-to-lot. Such deficiencies can cause uneven polishing, scratches or other related anomalies. This, in turn, can cause a number of yield and reliability problems.
Certain conventional systems have attempted to address such issues by monitoring the CMP process to determine when the slurry has degraded past an unacceptable point. Depending upon which conventional system is used, the slurry may then be discarded and replaced, or put through some sort of re-mixing process. Typically, re-mixing is of limited effectiveness, and both approaches introduce a high level of labor and material costs to the manufacturing process.
Conventional point-of-use mixing also causes certain issues for manufacturers. In certain conventional systems, point-of-use mixing is achieved by applying separate slurry components to the wafer while polishing—allowing the polisher (i.e., pad) to mix the components along the surface of the wafer. Utilizing such an approach, a manufacturer cannot be sure that a full and consistent mixing, or an even polish, has occurred.
Other conventional point-of-use mixing systems may rely on an apparatus to briefly mix a small quantity of components immediately prior to application from a single outlet (e.g., nozzle). In many cases, such mixing involves some sort of mechanical agitation (e.g., stirring, shaking) that causes physical impact within or between the components of the slurry. Depending upon the slurry components used, this impact stress can cause component shear—resulting in the formation of additional agglomerations and other non-conformities in the slurry. Furthermore, such mixing is usually brief—not allowing enough time for a full mixing or beneficial pre-reaction of slurry components (i.e., conditioning), depending upon the components used.
These and other similar conventional approaches thus result in a number of CMP irregularities. Agglomerations, gels, settled particulate matter and other non-conformities occurring in such conventional systems could overpolish, under polish, mar or scratch device structures on a wafer surface. If recognized, additional efforts or expenses must be incurred to compensate or allow for these irregularities. If not recognized, these irregularities detrimentally skew the CMP process. Either way, product yield and process costs are negatively impacted.
Moreover, such conventional systems—whether premixed or point-of-use—are typically static in nature, providing for only a single slurry formulation during CMP. Most conventional systems provide the ability to change slurry formulation only on a lot-to-lot basis. Depending upon the system utilized, a manufacturer might be able to change slurry formulation on a wafer-to-wafer basis. Given the methods and apparatus of most conventional systems, however, wafer-to-wafer modification would be an exceptional circumstance rather than a routine operation—as it would add a number of time and labor-intensive steps to the process (e.g., stopping processing lines, purging component reservoirs and conduits, refilling). For similar reasons, intra-wafer modification of slurry formulation would be commercially impractical, if not impossible.
As a result, there is a need for a versatile system for point-of-use mixing or conditioning of multi-component slurries used in CMP—a system that fully and completely mixes slurry components in a versatile, non-stressful manner that reduces or eliminates slurry nonconformities, and provides for real-time modification of slurry composition, in an easy, efficient and cost-effective manner.